Originally Posted by andrewwynn

Do you have a scanner that will pull the battery level histogram? It's a very good way to get a sense of is the problem more related to the battery or the starter.

Typical starter failure presents with the symptom you describe. Just takes longer to start. A defect in the starter will cause it to take much more current. It drops the voltage from the battery getting less torque at the starter, lower RPM.

Get a current reading on the starter cable and a load test on the battery